1. Field
The present invention relates to communications systems, and more specifically, to systems and techniques for synchronizing a communications device to an asynchronous network access point.
2. Background
Modern communications systems are designed to allow multiple users to share a common communications medium. One such communications system is a code division multiple access (CDMA) system. A CDMA communications system is a modulation and multiple access scheme based on spread-spectrum communications. In a CDMA communications system, a large number of signals share the same frequency spectrum and, as a result, provide an increase in user capacity. This is achieved by transmitting each signal with a different code that modulates a carrier, and thereby, spreads the spectrum of the signal waveform. The transmitted signals are separated in the receiver by a correlator that uses a corresponding code to despread the signal's spectrum. The undesired signals, whose codes do not match, are not despread in bandwidth and contribute only to noise.
In a CDMA communications system, a user may access a network, or communicate with other users, through a network access point. A network access point generally includes a radio network controller supporting multiple nodes. For the purpose of this disclosure, the term “node” will be used to refer to a node B, a base station, or any other similar communications station. Each node is assigned to serve all users within a region generally referred to as a cell or sector. Within any given region, a user may be in communication with any number of neighboring nodes as well as the node serving the region.
In some CDMA communications systems, the nodes are synchronized to one another. By way of example, the Navstar Global Positioning satellite navigation system is often used to synchronize the nodes to a common time reference. As a result, once a user acquires and synchronizes to a node, it can synchronously communicate with other nodes as it travels from region to region. This is to be contrasted to an asynchronous CDMA communications system which may require the user to resynchronize to different nodes as it travels through various regions of coverage. The resynchronization process should be performed quickly to minimize potential interruptions in communications that may be perceived by the user. Furthermore, it would be advantageous to minimize the time during which a user is performing the resynchronization process because this reduces the risk of dropped radio communication links to nodes other than the reference node, and it makes position estimates that are based on propagation delay measurements more accurate.